A proposal has been made of a technology that monitors an alertness level of a driver of a vehicle. When a decrease in the alertness level of the driver is detected, a warning message is output to the driver for the purpose of avoiding occurrence of an accident (patent literature 1). According to the proposed technology, an image of the driver's face is analyzed in order to detect a feature manifested on the face when the alertness level has decreased, and the alertness level is estimated based on a result of the detection. Further, when the alertness level is estimated, information indicating the reliability of the alertness level is detected. This makes it possible to output an appropriate warning message to the driver.
A technology for measuring an alertness level by detecting a driver's electroencephalographic signal has been proposed (patent literature 2). The electroencephalographic signal is a signal resulting from a brain activity. Thus, when the alertness level is measured based on the electroencephalographic signal, the alertness level can be directly measured instead of indirectly estimating the alertness level by detecting a feature manifested on a face. Therefore, the alertness level is expected to be detected highly precisely with a higher reliability.
However, the method of detecting a driver's electroencephalographic signal is confronted with a problem that an instrument for detecting the electroencephalographic signal has to be attached to the driver's head. The electroencephalographic signal is a substantially feeble signal and is easy to be affected by noise. This poses a problem that it is not easy to ensure precision and reliability in detection of an alertness level of the driver.